Lets just say your data and my data disagree....
The reason people don't run a MPSC at autocross is because of temp issues. You cannot get the tire hot under normal autocross conditions.
Pinching a tire, running too wide for the rim, generally doesn't generate more grip. However, this varries from tire to tire and hoosiers generally handle this better than most, because in part they are designed to run on narrower rims. However, for everything there are limits and 265 on 8.5" is really close. You have to think about what happens to a tire when the sidewall deflect during cornering.
Hoosiers are not 4s faster than PS2s on a 60 second course. There will also be variations dependant on surface conditions.
Originally Posted by M3Rad
Actually there is a better option on 19", 265/35 and 285/30.
With the proper tire pressure the 275 work with the stock wheels. The rounded shoulder (running a too wide tire for the wheel) helps camber challenged cars. I ran 275 RA1/V700 on 8.5" wheels with better results (lap times) than stock 235 in a 911.
Hoosiers A6 are around 4 secs faster on a 60 secs course. The stock car doesn't have race tires, it has Michelin PS2.
At a 60 secs track, my GT3 was 1.5 secs faster on MPSC compared to MPS2. If you were right, a lot of people would be running MPSC for auto-x given just that 0.5 secs difference against Hoosier A6 (0.25 on a 30 secs course), and way longer life, plus the ability to drive them on the streets.
The E46 M3 CSL gained around 20 secs at the Nurburgring by switching to MPSC, the 2007 GT3 gained over 10 secs at the same track by going to a street friendly version of the MPSC.
I have run R6 and they have much higher grip levels than the MPSC, around 1 sec faster on the same 60 secs track (Pocono North). I also ran Michelin slicks (S6/S7) and A6 for auto-x/track, at auto-x the A6 is faster, at the track they are even for the first 2 laps, after that the A6 goes greasy. There are multiple comparisons between R6 and real slicks from SCCA/PCA/NASA Club Racing.
I personally run half the event on my street tires when I run A6 at local events, so I get longer life out of the A6, and I keep comparing lap times.